DESCRIPTION: Medulloblastoma, the most common malignant brain tumor in childhood, continues to represent a therapeutic challenge. Although the majority of children with standard-risk (localized resectable tumors without metastases) are cured with surgical and radiotherapeutic intervention, 30 percent of these patients relapse and die. Children with high-risk medulloblastoma (extensive or metastatic tumors) do far worse, with the majority failing treatment with surgery, radiotherapy and chemotherapy. Demonstration of the activity of bifunctional alkylators such as cyclophosphamide in the treatment of medulloblastoma has provided the opportunity for advances in the treatment of high- risk medulloblastoma, reduction of the dose (and presumably neurotoxicity) of radiation used to treat standard-risk medulloblastoma, and the salvage of patients with recurrent medulloblastoma. Review of recent clinical trials using cyclophosphamide for treatment of medulloblastoma now indicates two major impediments to further progress: emergence of drug-resistant tumor cells and spread of tumor cells to the leptomeninges. Resistance to alkylating agents, including cyclophosphamide, is multifactorial, with a diverse spectrum of mechanisms observed in murine and human neoplasia, including increased aldehyde dehydrogenase activity, increased GST activity and elevated levels of GSH. Mechanisms of tumor resistance to 4- hydroperoxycyclophosphamide (4-HC) have been studied using a panel of human medulloblastoma cell lines with either laboratory-generated resistance to 4-HC or established from tumors showing clinical resistance to cyclophosphamide. Recent studies indicate preferential repair of DNA interstrand crosslinks (ICL) in a resistant line compared to a parental line, supporting further studies designed to define the scope and relevance of DNA ICL repair as a mechanism underlying resistance of medulloblastoma to cyclophosphamide. Medulloblastoma has a marked tendency for subarachnoid dissemination, with the incidence of leptomeningeal involvement at diagnosis approximately 30 percent using both cytologic analysis of cerebrospinal fluid and radiographic imaging. Recent studies demonstrating the activity of busulfan (as opposed to other alkylators) against 4-HC-resistant medulloblastoma cell lines, coupled with initial experiments indicating that busulfan can be administered intrathecally producing CSF levels > 3 logs higher than that produced by oral administration at a dose used for bone marrow transplantation suggest that intrathecal busulfan may prove effective in prophylaxis or treatment of leptomeningeal medulloblastoma. The specific aims of this proposal are: 1) To construct a phosphoramide mustard- induced single DNA ICL in a defined position of a plasmid vector; 2) To define the molecular events mediating repair of phosphoramide mustard-induced DNA ICL; 3) To define the role of repair of phosphoramide mustard-induced DNA ICL in mediating cyclophosphamide resistance in medulloblastoma.